The present invention relates generally to bicycle systems, and frame assemblies. In particular, the present invention relates to a bottom bracket shell assembly for an alloy bicycle frame.
Bicycle frames traditionally comprise a plurality of tubes, including a head tube, a top tube, a down tube, a seat tube, chainstays and seat stays. A bottom bracket shell commonly connects the seat tube, down tube and chainstays and receives a bottom bracket. The bottom bracket receives a crankset of a drivetrain for rotational movement, which transforms forces created by the rider during pedaling into power to drive the rear wheel. The bottom bracket shell functions as the central support for the forces created by the rider, including the gravitational force transferred from the seat tube and down tube and the pedaling forces transferred from the crankset.
The bottom bracket shell in prior art alloy bicycle frames traditionally comprises a metal tube which is oriented transverse to the other tubes of the frame. The down tube and seat tube commonly have mitered ends which are configured to wrap around the bottom bracket shell. The seat tube and down tube are often welded to the circumference of the bottom bracket shell in order to form a contiguous assembly.
The welding process involves forming a joint by melting sidewalls of parent tubes and simultaneously replenishing the joint with additional material through the use of a filler rod. This process is heavily dependent on the welding operator's abilities and, because of this, the size, shape and smoothness of a weld will vary in consistency depending on the talents of the welding operator. This means that the structural strength of the bicycle frame can vary depending on the abilities of the welding operator.
Due to the relatively large diameter of the seat tube, down tube and chainstays versus the relatively small circumference of the bottom bracket shell, it is not possible to completely attach each tube directly to the bottom bracket shell. The diameters of the tubes are limited in order to fit the down tube and the seat tube on the bottom bracket shell. The diameters of the tubes are also limited because connecting the tubes near the lateral edge of the bottom bracket shell increases the risk that structural portions of the bottom bracket shell will be unintentionally burned away during the welding process. The limit on the diameters of the tubes reduces the material available to strengthen the bottom bracket, however. When the down tube and seat tube are formed from low modulus alloys such as aluminum, the thickness of sidewalls of the tubes is undesirably increased to provide the requisite structural strength to the bicycle frame. Moreover, the limited diameter of the seat tube and down tube in relation to the width of the bottom bracket shell requires that several welds be placed on top of each other. Consequently, the configuration of prior art alloy bicycle frames limits the strength to weight ratio of the bottom bracket shell.
Accordingly, there is a need in the art for an improved bottom bracket shell having an increased strength to weight ratio.